1. Field of the Invention
The present invention relates to a control method for an automatic transmission and a system therefor, which can judge whether a rotation speed sensor for detecting rotation speed of an input shaft and an output shaft of an automatic transmission is in normal condition or abnormal condition.
2. Description of the Related Art
In an automatic transmission, it is required to progressibly vary supply pressure for frictional engaging element newly engaging upon shifting operation. At the same time, an output torque of an engine is temporarily lowered for reducing variation of torque which can be caused upon shifting operation and thus achieving smooth shifting operation without causing shift shock. Therefore, there is generally employed a method for appropriately adjusting a supply pressure for frictional engaging element by means of a line pressure controlling valve or so forth, on the basis of detection signals detected by employing an input shaft rotation speed sensor detecting rotation speed of an input shaft of the automatic transmission and an output shaft rotation speed sensor detecting rotation speed of an output shaft of the automatic transmission.
However, when failure is caused on the input shaft rotation speed sensor or the output shaft rotation speed sensor, it becomes impossible to perform appropriate control for reducing shift shock as set forth above. Therefore, in the prior art, when failure is caused in the input shaft rotation speed sensor or the output shaft rotation speed sensor, an appropriate gear ratio is set or a supply pressure for the frictional engaging element is set in order to avoid problem in a control system. Such fail-safe control system for the automatic transmission has been proposed in Japanese Patent Application Laid-open No. 97765/1990.
A fail detection system for an automatic transmission as disclosed in Japanese Patent Application Laid-open No. 97765/1990 may detect any failure caused on the input shaft rotation speed sensor or the output shaft rotation speed sensor.
However, in such conventional systems, it is not possible to identify which of the input shaft rotation speed sensor or the output shaft rotation speed sensor is the faulty sensor. Therefore, it becomes difficult to optimally perform the fail-safe process. Therefore, the fail-safe process is inherently performed under assumption that all sensors are in faulty condition. For instance, despite of the fact that failure is caused only in the input shaft rotation speed sensor, a gear ratio of the automatic transmission is held at third speed ratio, in which the input shaft and the output shaft of the transmission are directly coupled for rotation at equal speeds, and the supply pressure (line pressure) to be supplied to the frictional engaging element which becomes engaged upon shifting is adjusted to the maximum value. Therefore, it inherently cause degradation of riding quality, and an unnecessarily long period is required for restoration from this condition.
On the other hand, since the faulty rotation speed sensor cannot be identified, it is also difficult to store the individual fault history of the rotation speed sensor.